1. Field of the Invention
This invention relates generally to control systems and more specifically to converter control of variable-speed wind turbines.
2. Background
Wind power is today's most rapidly growing renewable energy source. Large-scale wind generation facilities have become a very visible component of the interconnected power grid in many portions of the United States and around the world. Wind turbines can operate at either fixed speed or variable speed. For a fixed-speed wind turbine, the generator is directly connected to the electrical grid. For a variable speed wind turbine, the generator is controlled by power electronic equipment. The two most-common variable speed wind turbines are wind turbines that use direct-driven synchronous generators (DDSG) or double-fed induction generators (DFIG). For both of them, a frequency converter such as a pulse-width modulation (PWM) AC/DC/AC frequency converter is connected between the grid and the generator. FIG. 1A illustrates an embodiment of an AC/DC/AC converter in modern variable-speed synchronous generator wind turbine, and FIG. 1B illustrates an embodiment of an AC/DC/AC converter in variable-speed DFIG wind turbine. For a DDSG wind turbine, the converter is connected between the generator and the grid, as shown in FIG. 1A, and for the DFIG wind turbine, the converter is connected between the rotor circuit and the grid, as shown in FIG. 1B.
A doubly-fed induction generator is a standard, wound rotor induction machine with its stator windings directly connected to the grid and its rotor windings connected to the grid through an AC/DC/AC frequency converter (FIG. 1B). In modern DFIG designs, the frequency converter is comprised of two self-commutated PWM converters, a machine-side converter and a grid-side converter, with an intermediate DC voltage link. The converter connected to the induction rotor circuit is the machine-side converter, and the converter connected to the grid is the grid-side converter. Traditionally, each of these two PWM converters is controlled by using decoupled d-q control approaches, as are known in the art. Basically, the machine-side converter controls the real and reactive power production of the induction machine, and the grid-side converter controls the dc-link voltage and the reactive power absorbed from the grid by the converter. The general control technique for the grid-side converter control, which is widely used in wind power industry, is a decoupled d-q control approach that uses the direct (d) axis current component for real power control and quadrature (q) axis current component for reactive power control. By controlling the converters on both sides, the DFIG characteristics can be adjusted so as to achieve maximum of effective power conversion or capturing capability for a wind turbine and to control its power generation with less fluctuation. However, to meet these needs, the grid-side converter should be controlled in such a way to maintain a constant DC-link capacitor voltage and to keep the converter operation at a desired power factor.
Therefore, what is desired are control systems and methods that overcome challenges present in the art, some of which are described above.